As the capacity of recording apparatuses using magnetic recording media increases, efforts have been made to reduce the magnetic head flying height so that the recording density can be increased. To reduce the magnetic head flying height, a magnetic recording medium having excellent smoothness, i.e., a magnetic recording medium having high surface precision is required. For example, in the case of non-magnetic metal (i.e., Al, etc.) substrates that have been widely used in the past, high precision machining is required.
Following is an example of a method of manufacturing a magnetic recording medium substrate comprising a conventional non-magnetic metal substrate, and a magnetic recording medium using that substrate. For the non-magnetic metal substrate, in general a blank is obtained by subjecting a heated molten metal material to rolling and hot annealing, and then machining to prescribed. The blank is subjected to inside/outside diameter processing, and lapping is carried out to improve the surface precision. After that, a 13 μm Ni—P plating layer is formed on the blank to improve the surface hardness and so on. The surface of the Ni—P plating layer is polished to a surface roughness Ra of 10 Å, and then the polished surface is subjected to final lapping using a diamond slurry. The substrate obtained is subjected to laser zone texturing such that, for example, the bump height becomes 190 Å and the bump density becomes 30×30 in a contact start/stop (CSS) zone. Next, the substrate is subjected to precision cleaning, thus obtaining a magnetic recording medium substrate.
A 500 Å Cr foundation layer, a 300 Å Co-14Cr-4Ta magnetic layer, and an 80 Å carbon protective layer are formed in this order by DC sputtering on the magnetic recording medium substrate. After the sputtering, the surface is subjected to tape burnishing, and then a 20 Å fluorinated lubricant layer is formed by dip coating or spin coating to form a finished magnetic recording medium.
The conventional method of manufacturing a magnetic recording medium substrate and a magnetic recording medium described above has become more complicated in recent years, accompanying demands for further increases in recording density. Furthermore, there are also demands to provide magnetic recording media that are cheaper than conventional ones and yet still give a higher performance. Magnetic recording media in which a thermoplastic resin is used for the magnetic recording medium substrate have been proposed as magnetic recording media for meeting these demands.
A method where a plastic magnetic recording medium substrate is manufactured using a molding technique, and a CSS zone and an undulating pattern that serves as a preform for servo marks on the magnetic recording medium are formed simultaneously during the molding gives excellent productivity and is industrially favorable, and magnetic recording media can be provided inexpensively. For example, Japanese Patent Application Laid-open No. 7-153060 (paragraphs 0009 to 0010) discloses a magnetic recording medium substrate that is formed by injection molding a thermoplastic norbornene type resin and having an optimized thickness, and a prescribed shape on the surface thereof.
Moreover, as another example, Japanese Patent Application Laid-open No. 7-210855 (paragraphs 0009 to 0010) discloses a magnetic recording medium obtained by injection molding a magnetic disk substrate from a filler-containing plastic composite material for which the types and contents of the filler and the matrix resin, the particle diameter of the filler, the thickness of the magnetic disk substrate and so on have been optimized, and then providing a magnetic layer on the substrate.
Furthermore, as another example, Japanese Patent Application Laid-open No. 9-85743 (paragraph 0005) discloses a method of obtaining a molded article, especially a substrate used in a magnetic recording medium, having excellent surface precision by injection molding a thermoplastic norbornene type resin at a prescribed temperature and over a prescribed time, and the molded article obtained.
A plastic magnetic recording medium substrate manufactured by, for example, injection molding synthetic resin pellets, when compared with a metallic substrate or a ceramic substrate made of glass or the like, the mechanical strength properties, such as tensile strength and the tensile and bend elastic moduli, are generally poor. Hence, there are problems, such as the flatness of the substrate dropping due to stress arising during molding or release of the substrate from the mold, undulating defects of magnitude several μm arising, and the minute waviness becoming large, associated with injected molded substrates. Furthermore, when the plastic magnetic recording medium substrate has a high coefficient of thermal expansion, shape change will occur at a high-temperature environment. Furthermore, in the case of a highly hygroscopic substrate, the change in shape will be further promoted. These conditions create obstacles for forming a magnetic recording medium substrate that is required to have high surface precision.
If a magnetic recording medium is manufactured using a magnetic recording medium substrate having surface defects, reduced flatness, and large minute waviness, then reading and writing with the head may not be possible, and in particular during continuous or high speed seeking using a head with a low flying height, the traveling of the head will become unstable, leading ultimately to head crashes, and reliability issues.
Moreover, even when a plastic magnetic recording medium substrate has prescribed shape properties and surface precision, under a high-temperature, high-humidity environment (e.g., 500 hours at 80° C. and 80% relative humidity (RH)), the change in shape of the substrate will increase, leading again to the problems described above.
The plastic magnetic recording medium substrate can be formed with a polycarbonate resin, a polymethyl methacrylate resin or the like, and a thermoplastic norbornene type resin. However, polycarbonate resins and polymethyl methacrylate resins used for optical disk substrates deform through moisture absorption or heat, making it unsuitable. Moreover, although a thermoplastic norbornene type resin has relatively good properties with regard to heat resistance, hygroscopicity, and shape stability, such a resin again has problems described above.
Accordingly, there is a need for a plastic magnetic recording substrate that has good mechanical properties suitable for a high density magnetic recording medium. The present invention addresses this need.